The present invention relates generally to radio frequency (RF) signal power amplifiers, and more particularly to an improved impedance matching circuitry for RF signal power amplifiers utilizing microstrip transmission line techniques.
In prior art RF signal power amplifiers utilizing microstrip transmission line techniques, impedance matching is typically accomplished by utilizing capacitive or inductive stubs located at predetermined distances from the amplifying devices for terminating the amplifying device with a predetermined output impedance. The location of the capacitive and inductive stubs may be determined by utilizing conventional transmission line techniques, such as, for example, the single-stub matching techniques described on pages 94-96 of the text, "Transmission Lines and Wave Propagation," written by Philip C. Magnusson and published by Allyn and Bacon, Inc., 1965. These stubs are spacially located at right angles to the transmission line and are electrically coupled in parallel with the transmission line. In addition, two or more such stubs may be connected opposite one another in a wing-like arrangement. Furthermore, the pair of stubs need not be at right angles with respect to the transmission line. Pairs of such stubs, commonly referred to as "capacitance wings" have been utilized in Motorola Instruction Manual No. 68P81030E10, entitled "Cell Site RF Power Amplifier," published by Motorola Service Publications, Motorola, Inc., Schaumburg, Ill. 1975.
These stubs may be added to the output microstrip circuitry of an RF signal power amplifier for impedance matching transistors of the amplifier to a predetermined output impedance in order to avoid excessive standing voltage and current waves due to impedance mismatches. Such impedance mismatches can result from tolerance variations encountered in the manufacture of the microstrip circuitry of RF signal power amplifiers and from failures in succeeding amplifying stages. When subjected to impedance mismatches, transistors in RF signal power amplifiers may become unstable and regeneratively oscillate. The spurious signals created by such regenerative oscillations produce undesirable interference on the amplified RF signal. When subjected to impedance mismatches, the foregoing stub matching techniques do not adequately suppress the spurious signals resulting from such regenerative oscillations of the amplifier transistors. Thus, there has been a long felt need for improved impedance matching circuitry for RF signal power amplifiers that prevents regenerative oscillations when the amplifying devices are terminated by impedance mismatches.